我有两个 model Store has_many :dishes Dish belong_to :store

我用 session 来保存购物车 session[:cart] ||= Cart.new

class Cart
  attr_reader :items, :key_items
  def initialize
    @items = Hash.new
    @key_items = Hash.new
  end

  def add_dish(dish)
    #binding.pry
    if @key_items[dish.store_id].nil?
      @items[dish.store_id] = Array.new
      #使用dish.store报错没找出为什么
      @key_items[dish.store_id] = dish.store
    end

    current_item = @items[dish.store_id].find{|item| item.dish == dish}

    if current_item
       current_item.increment_quantity
    else
      @items[dish.store_id] << CartItem.new(dish)
    end
  end
end

其中 add_dish(dish) 中的 @key_items[dish.store_id] = dish.store 就会出错 用 @key_items[dish.store_id] = Store.find_by_id(dish.store_id) 就不会出错

出错如下，会破坏 session 中结构 正确的 session[:cart] 的值为

{"cart"=>
  #<Cart:0x007f9ffc2dc780
   @items=
    {1=>
      [#<CartItem:0x007f9ffc2dc6b8
        @dish=
         #<Dish id: 1, store_id: 1, name: "红烧肉", count: 30, remaining_count: 30, price_cents: 1100, price_cents_currency: "CNY", image: nil, description: nil, created_at: "2012-10-11 07:38:26", updated_at: "2012-10-11 07:38:26">,
        @quantity=1>,
       #<CartItem:0x007f9ffc2db380
        @dish=
         #<Dish id: 2, store_id: 1, name: "白切鸡", count: 30, remaining_count: 30, price_cents: 1100, price_cents_currency: "CNY", image: nil, description: nil, created_at: "2012-10-11 07:38:26", updated_at: "2012-10-11 07:38:26">,
        @quantity=1>]},
   @key_items=
    {1=>
      #<Store id: 1, name: "IT厨房", created_at: "2012-10-11 07:38:26", updated_at: "2012-10-11 07:38:26", start_price_cents: 2500, start_price_cents_currency: "CNY", fare_cents: 800, fare_cents_currency: "CNY">}>,
 :@stale_state=>nil}

错误的值为

{"cart"=>
  #<Cart:0x007fa8742deeb0
   @items=
    {1=>
      [#<CartItem:0x007fa8742dedc0
        @dish=
         #<Dish id: 1, store_id: 1, name: "红烧肉", count: 30, remaining_count: 30, price_cents: 1100, price_cents_currency: "CNY", image: nil, description: nil, created_at: "2012-10-11 07:38:26", updated_at: "2012-10-11 07:38:26">,
        @new_record=false>]},
   @quantity=1>,
 :@key_items=>
  {1=>
    #<Store id: 1, name: "IT厨房", created_at: "2012-10-11 07:38:26", updated_at: "2012-10-11 07:38:26", start_price_cents: 2500, start_price_cents_currency: "CNY", fare_cents: 800, fare_cents_currency: "CNY">}}

大家可以看到错误的值里面多了 @new_record=false 所以破坏了对象结构，所以无法还原对象

请问是怎么回事啊？我查了很久了